1. Field of the Invention
The invention pertains generally to systems for sensing or indicating an abnormal condition of a vehicular tire and is more particularly directed to such systems using radio frequency transmissions for signaling.
2. Description of the Prior Art
In the prior art numerous devices and apparatus have been suggested for sensing or indicating an abnormal condition in a vehicular tire. These include low pressure devices that indicate underinflation and minimum pressure devices that indicate a substantially complete loss of air or a flat.
One method of sensing an abnormal tire condition has been to provide a fluid pressure sensitive switch inside the tire and attached to either the wheel rim or tire sidewall. As the abnormal condition, such as a drop or loss of fluid pressure, develops the switch will operate to provide a warning signal. This warning signal can subsequently be communicated to a receiver monitor close to the operator of the vehicle to alert him of the condition.
Problems incurred with the fluid pressure sensitive switches are numerous in that they are mechanical and need adjustment and are not as reliable as one may prefer. For example, the opening communicating the air pressure to the device may become clogged or the spring actuator may become rusted because of moisture entering the device. The fluid pressure sensitive switch is not rugged enough to provide totally reliable service in the exposed locations that vehicle tires must operate in.
This is especially true for those fluid pressure devices which attach to the valve stems of the vehicle where no protection whatsoever is afforded. Another problem inherent with the valve stem operated pressure sensitive device is the provision of an additional place from which to leak air. Thus, they may contribute to the very condition the apparatus was attached to warn against.
The signals from the pressure operated switches have, in the past, been communicated to a centrally located monitor by either radio wave or contacting wires. If contacting wires are used, the problem of transmitting a signal from a rotating device presents itself. This has been accomplished laboriously in the past with slip rings or the like. On the other hand and if radio waves are used, there are the difficulties of powering the transmitter and not interferring with other communication facilities that may be nearby. Many of the transmitter indicators include batteries that have a limited operational cycle and require replacement at certain intervals.
A further problem having to do with previous indicators is that they are not particularly well adapted to the recent developments concerning "run-flat" tires. The new run-flat tires generally provide a small inner insert having a load bearing surface on which a vehicle may be driven for a reasonable time and speed while the tire is deflated. The new "run-flat" designs therefore, eliminate the need for spare vehicle tires but increase the difficulty of determining when a tire has lost its pressure.
It would be advantageous to provide a deflated tire condition indicating apparatus overcoming the problems faced in the prior art and which could also be easily integrated with a "run-flat" insert.